The use of a lubricating oil composition having high viscosity leads to disadvantages such as an increased energy loss caused by a viscosity and an increased stirring loss of the lubricating oil composition. In order to overcome such disadvantages and achieve energy savings, low-viscosity lubricating oil compositions have been recently introduced in the field of lubricating oils. The low-viscosity lubricating oil compositions, however, suffer evaporation loss especially when used under a high-temperature atmosphere. In order to solve such a problem, there has been an increasing demand for a low-viscosity base oil that meets the requirement for low evaporability.
As lubrication base oils having low viscosity and suffering little evaporation, ester-based base oils are known. However, the ester-based base oils are poor in moisture resistance. That is, the ester-based base oils yield acids on hydrolysis with moisture in the air when they are used for a long term. The yielded acids may result in metallic corrosion. Further, the ester-based base oils, because of their high polarities, have adverse effects on organic materials such as rubbers and plastics.
In recent years, visual devices, audio devices, personal computers, etc. have been reduced in size and weight and increased in memory capacity, and increased in information processing speed. In keeping with this trend, fluid bearings have been developed and commercialized for their applications to rotating devices for driving magnetic disks and optical discs, such as FD, MO, mini disc, compact disc (CD), DVD and hard disk, employed in those electronic devices. The fluid bearing, which is made up of a sleeve and a rotation shaft facing each other via a lubricant, has no ball bearing. As such, the fluid bearing is suitable for reducing the size and weight of an electronic device and is excellent in, for example, silence and economical efficiency. A lubricating oil used for such a fluid bearing is required to have low viscosity even in a low-temperature range and to have (i) excellent low-temperature fluidity, (ii) a viscosity property of suffering little viscosity decrease even in a high-temperature range, and (iii) low evaporability.
Thus, in the field of lubricating oils, there has been an increasing demand for lubrication base oils and lubricating oil compositions both of which have high moisture resistance and physical properties such as low viscosity, low evaporability, low-temperature fluidity, and a high viscosity index.
In order to enhance performance of a lubricating oil, the mixing of an additive such as an antioxidant is generally performed. For example, Patent Literature 1 discloses that phenyl-α-naphthylamine, p,p′-dialkyl diphenylamine, and phosphate ester are mixed into a base oil in order to improve thermal stability, oxidative stability, resistance to sludge, lubricity, longevity, and water separability of a lubricant. Patent Literature 2 discloses a watch lubricating oil containing (i) an ether oil that serves as a base oil, (ii) an anti-wear agent, and (iii) an antioxidant, as a lubricating oil composition which is operable in the temperature range of −30° C. to 80° C., is free from change of properties over a long period of time, enables a life of watch battery to last long, and is favorable as a watch lubricating oil.